Hi there, I’m having much the same problem but would seem to be due to some unpublished rate limit and penalization rules. I have a small app that will post members lists/members/create_all and if that fails will revert to lists/members/create to submit members individually.
What I’ve noticed is that after one or two list updates I get the above error. There’s no mention of rate limits beyond the 403 http error which indicated it’s an authorization error.
I’ve created a fresh application just in case and still get the same issue.
Now, what makes this scary is if I log into twitter with the same accounts I can’t add members to existing or new lists - however if I log in with a virgin account I can - therefore those accounts are locked from adding members regardless of the application - I imagine this lock might go after a day but should that really happen at all. In one test around 80 members were added, in the new application two lists of around the same were added before lock down.
The only thing I can think is maybe members are being added too quickly? And therefore trigger some alert?
(the list is owned by the account adding members, the list is nowhere near full, not blocked)